Quality standards in the specialty coffee industry established during the 1950's and 1960's that coffee extraction of approximately 20% (usually a range of ˜18-22%) will achieve the best quality brewed coffee, using various brew methods. Over the ensuing years these established standards have been generally referred to as the gold-cup standard, and have been accepted internationally by many institutions, educational establishments, and standards committees. The precise extraction (solubles yield) and strength (solubles concentration) may be varied for particular coffees, to achieve finely tuned recipes for particular coffee cultivars, climates and growing regions, also known as terroir, and other characteristics.
Extraction (solubles yield) refers to the percentage of the dry coffee by weight that is removed by dissolving in water during the brewing process. Up to 30% of the available soluble solids in ground coffee can be extracted, with most of the remaining 70% being cellulose, and not soluble in water. However, generally speaking, extracting more than 22% will begin to sharply increase those components in coffee that contribute to the bitter taste defects associated with over-extraction. Extracting less than 18% is generally associated with weak, under-developed taste defects.
The finished brew is a balance between extraction (solubles yield) and strength (solubles concentration). Over-extracting results in the extraction of many of the bitter components of the remaining solids. These contribute significantly to those taste defects known as bitter, strong-bitter and weak bitter. Under-extracting causes a taste defect that is under-developed and can be weakly or strongly under-developed. As coffee dissolves, the bulk of the solids including the sweeter components dissolve during the first 30-50% of the brewing cycle, and the more bitter elements start to dissolve in the latter half of the brewing cycle. The highest quality coffee will have rich aroma, fullness of body, delicacy of flavor as well as clarity and unique character—all optimally balanced. Achieving these attributes requires a significant degree of precision during the coffee brewing process.
Strength (solubles concentration) refers to the measured amount of solids extracted into the final coffee solution. Strength may be expressed as the percentage total dissolved solids (% TDS). For example, for 100 grams of a coffee measuring 1.3% TDS, there are 98.7 grams of water, and 1.3 grams of dissolved coffee solids in solution.
Refractive index measurements have been used for process control in the food industry since the 1940s. Typical measurements are usually for sugars in fruits such as melons, for orange and other juices, for sugar content in grapes for the wine industry, and many other examples.